With an increasingly scaling down of a feature size of MOSFET (metal-oxide-semiconductor field-effect transistor), a low carrier mobility of Si has become a primary factor restraining a performance of devices. In order to solve the problem, a material with higher mobility is adopted as the channel material, for example, Ge is adopted as the channel material in PMOSFETs, and a group III-V compound semiconductor material is adopted as the channel material in NMOSFETs. A hole mobility of Ge is around four times as great as that of Si, and currently a lot of technical challenges for a Ge channel MOSFET have been overcome. A group IV semiconductor material Ge-based Ge1-xSnx (GeSn) compatible with Ge has a good electrical property. Firstly, since the strained GeSn has a greater hole mobility than Ge, the strained GeSn has a good application prospect on a channel of PMOSFET; secondly, a uniaxis compressive strain may be introduced in a Ge channel of a MOSFET device by filling a strained Ge1-xSnx (0<x<1) (GeSn) alloy in a source region and a drain region, thus greatly improving a performance of the Ge channel MOSFET, especially when a length of the Ge channel is on a nanometer scale; thirdly, according to a theoretic research, the strained Ge1-xSnx (x>0.11) alloy will be a direct bandgap semiconductor with a good opto-electrical property. Furthermore, the GeSn alloy is compatible with a conventional silicon CMOS (complementary metal oxide semiconductor) process.
However, it is difficult to directly grow a GeSn alloy with high crystalline quality and high Sn content on a Ge substrate. The reasons are illustrated as follows. Firstly, an equilibrium solid solubility of Sn in Ge is less than 1% (i.e., about 0.3%); secondly, a surface segregation of Sn easily occurs because the surface energy of Sn is smaller than that of Ge; and thirdly, there is a large lattice mismatch (about 14.7%) between Ge and α-Sn. In order to suppress the surface segregation of Sn and increase the content of Sn, a certain amount of Si may be doped during a growth to form a Ge1-x-ySnxSiy (0<x<1, 0<y<1) (GeSnSi) layer. Because a lattice constant of Si is smaller than that of Ge, but a lattice constant of Sn is larger than that of Ge, a thermal stability of the GeSnSi alloy may be improved by doping Si into it.
It is difficult to fabricate GeSn and GeSnSi since both materials are metastable Ge-based materials. Molecular beam epitaxy (MBE) is conventionally used for growing the GeSn alloy. By using such a method, a GeSn film with high crystal quality may be obtained. Disadvantages (such as expensive equipment, time-consuming fabrication process and high cost) of such a method, however, limit a large scale production. In addition, a uniformity of the film formed by MBE needs to be further improved. Alternatively, chemical vapor deposition (CVD) is also used for growing the GeSn or GeSnSi film but has disadvantages of poor film quality, poor thermal stability, easy segregation of Sn and high cost.